The present invention relates to plasma processing method and apparatus typified by plasma doping for introducing impurities to a surface of a solid sample of a semiconductor substrate or the like.
As the technique for introducing impurities to a surface of a solid sample, there has been known a plasma doping method for ionizing and introducing impurities into a solid with low energy (see, e.g., U.S. Pat. No. 4,912,065). FIG. 13 shows an outlined construction of a plasma processing apparatus to be used for the plasma doping method as a conventional impurity introduction method described in above-mentioned U.S. Pat. No. 4,912,065. Referring to FIG. 13, a sample electrode 206 for placing thereon a sample 209 formed of a silicon substrate is provided in a vacuum chamber 201. In the vacuum chamber 201 are provided a gas supply unit 202 for supplying a doping raw material gas containing desired elements, e.g., B2H6, and a pump 203 for pressure-reducing the interior of the vacuum chamber 201, by which the interior of the vacuum chamber 201 can be maintained at a specified pressure. From a microwaveguide 219, a microwave is radiated into the vacuum chamber 201 via a quartz plate 207 as a dielectric window. By interaction of this microwave and a dc magnetic field formed from an electromagnet 214, an effective-magnetic-field microwave plasma (electron cyclotron resonance plasma) 220 is formed in the vacuum chamber 201. A high-frequency power supply 210 is connected to the sample electrode 206 via a capacitor 221 so that voltage of the sample electrode 206 can be controlled. It is noted that the gas supplied from the gas supply unit 202 is introduced into the vacuum chamber 201 through a gas inlet 211, and discharged to the pump 203 through a gas outlet 212.
In a plasma processing apparatus of such a constitution, a doping raw material gas, e.g., B2H6, introduced through the gas inlet 211 is formed into a plasma by a plasma generation means composed of the microwaveguide 219 and the electromagnet 214, and boron ions in the plasma 220 are introduced to a surface of a sample 209 by the high-frequency power supply 210.
On the sample 209 to which impurities have been introduced in this way, forming a metal interconnection layer, then forming a thin oxide film on the metal interconnection layer in a specified oxidizing atmosphere and thereafter forming a gate electrode on the sample 209 by a CVD device or the like allows, for example, MOS transistors to be obtained.
In this connection, a gas containing impurities that will become electrically active when introduced to a sample of a silicon substrate or the like, like the doping raw material gas made of B2H6, generally has an issue of high danger.
Also, in the plasma doping method, all the substances contained in the doping raw material gas are introduced to the sample. Referring to a doping raw material gas made of B2H6 as an example, although boron is the only effective impurity when the material gas is introduced to the sample, hydrogen is also introduced into the sample at the same time. With hydrogen introduced into the sample, there is a problem that lattice defects would occur to the sample during subsequent heat treatment such as epitaxial growth.
Then, it is conceivable that an impurity solid containing impurities that will become electrically active when introduced into the sample 209 is arranged in the vacuum chamber 201 and a plasma of a rare gas is generated within the vacuum chamber 201, so that the impurity solid is sputtered by ions of inert gas to separate impurities from the impurity solid (See, for example, Japanese Unexamined Patent Publication No. 09-115851.). FIG. 14 shows a schematic construction of a plasma doping apparatus used in a plasma doping method serving as an impurity introducing method of the prior art described in Japanese Unexamined Patent Publication No. 09-115851. In FIG. 14, a sample electrode 206 for placing thereon a sample 209 formed of a silicon substrate is provided in a vacuum chamber 201. In the vacuum chamber 201 are provided a gas supply unit 202 for supplying an inert gas, and a pump 203 for pressure-reducing the interior of the vacuum chamber 201, by which the interior of the vacuum chamber 201 can be maintained at a specified pressure. From a microwaveguide 219, a microwave is radiated into the vacuum chamber 201 via a quartz plate 207 as a dielectric window. By interaction of this microwave and a dc magnetic field formed from an electromagnet 214, an effective-magnetic-field microwave plasma (electron cyclotron resonance plasma) 220 is formed in the vacuum chamber 201. A high-frequency power supply 210 is connected to the sample electrode 206 via a capacitor 221 so that voltage of the sample electrode 206 can be controlled. An impurity solid 222 containing impurity elements, e.g., boron, is provided on a solid holding table 223, and voltage of the solid holding table 223 is controlled by a high-frequency power supply 225 connected thereto via a capacitor 224. It is noted that the gas supplied from the gas supply unit 202 is introduced into the vacuum chamber 201 through a gas inlet 211, and discharged to the pump 203 through a gas outlet 212.
In a plasma doping apparatus of such a construction, an inert gas, e.g., argon (Ar), introduced from the gas inlet 211 is formed into a plasma by a plasma generation means composed of the microwaveguide 219 and the electromagnet 214, and a part of impurity elements that have come out of an impurity solid 222 into the plasma by sputtering are ionized and introduced to the surface of the sample 209.
However, with the conventional method, there has been a problem that it would be hard to attain uniform doping of impurities to the sample surface.
FIG. 15 shows results of measuring sheet resistance in the doping of boron to a 200 mm-dia. silicon semiconductor substrate 209, where an x-axis is taken from above to below of FIG. 13, in the conventional plasma processing apparatus shown in FIG. 13. As apparent from FIG. 15, the sheet resistance is higher on one side closer to the gas inlet 211 (upper side in FIG. 13) and lower on another side closer to the gas outlet 212 (lower side in FIG. 13). This could be attributed to nonuniformity of the ion density of boron as an impurity source that would be due to effects of nonuniformities of the gas flow, in other words, nonuniformity of pressure, nonuniformity of flow velocity, nonuniformity of boron partial pressure, and the like.
In view of the above issues of the prior art, an object of the present invention is to provide a plasma processing method and apparatus capable of enhancing the processing uniformities of doping concentration and the like.